1. Field of the Invention
The present invention relates to an electro-optical device, typically an EL (electroluminescence) display device formed by a semiconductor element (an element using a semiconductor thin film) made on a substrate, and to electronic equipment (an electronic device) having the electro-optical device as a display (also referred to as a display portion).
2. Description of the Related Art
Techniques of forming a TFT on a substrate have been widely progressing in recent years, and developments of applications to an active matrix type display device are advancing. In particular, a TFT using a polysilicon film has a higher electric field effect mobility (also referred to as mobility) than a TFT using a conventional amorous silicon film, and high speed operation is therefore possible. As a result, it becomes possible to perform pixel control, conventionally performed by a driver circuit external to the substrate, by a driver circuit formed on the same substrate as the pixel.
This type of active matrix display device has been in the spotlight because of the many advantage which can be obtained by incorporating various circuits and elements on the same substrate in this type of active matrix display device, such as reduced manufacturing cost, small size, increased yield, and higher throughput.
Switching elements are formed by a TFT for each of the pixels in the active matrix display device, current control is performed by driver elements using the switching elements, and an EL layer (electroluminescence layer) is made to emit light. A typical pixel structure at this time is disclosed in, for example, in FIG. 1 of U.S. Pat. No. 5,684,365 (Japanese Patent Application Laid-open No. Hei 8-234683).
As shown in FIG. 1 of the U.S. Patent, a drain of a switching element (T1) is connected to a gate electrode of a current control element (T2), and is also connected in parallel to a capacitor (Cs). The gate voltage of the current control element (T2) is maintained by the electric charge stored in the capacitor (Cs).
Conversely, when the switching element (T1) is non-selected, the electric charge leaks through the switching element (T1) if the capacitor (Cs) is not connected (the flow of current at this point is referred to as off current), and the voltage applied to the gate electrode of the current control element (T2) becomes unable to be maintained. This is a problem which cannot be avoided because the switching element (T1) is formed by a transistor. However, the capacitor (Cs) is formed within the pixel, and therefore this becomes a factor in reducing the effective luminescence surface area (effective image display area) of the pixel.
Further, it is necessary for a large current to flow in the current control element (T2) in order to luminesce the EL layer. In other words, the performance required of the TFT becomes entirely different in the switching element and the current control element. In this type of case, it is difficult to ensure the performance required by all of the circuits and element with only one TFT structure.
In view of the above conventional technique, an object of the present invention is to provide an electro-optical device having good operation performance and high reliability, and in particular, to provide an EL display device. Another object of the present invention is to increase the quality of electronic equipment (an electronic device) having the electro-optical device as a display by increasing the image quality of the electro-optical device.
In order to achieve the above objects, the present invention assigns TFTs having an optimal structure in view of the performance required by elements contained in each pixel of the EL display device. In other words, TFTs having different structures exist within the same pixel.
Specifically, an element which places the most importance on sufficiently lowering the value of the off current (such as a switching element) is given a TFT structure in which the importance is more on reducing the off current value rather than on high speed operation. An element which places the greatest importance on current flow (such as a current control element) is given a TFT structure in which the importance is more on current flow, and on controlling deterioration due to hot carrier injection, which becomes a conspicuous problem at the same time, rather than on reducing the value of the off current.
It becomes possible to raise the operating performance of the EL display device, and to increase its reliability, with the present invention by performing proper use of TFTs on the same substrate, as above. Note that the concepts of the present invention are not limited to a pixel portion, and that the present invention is characterized by the point of being able to optimize the TFT structure contained in the pixel portion and in a driver circuit portion for driving the pixel portion.